1. Field of the Invention
The present invention is in the field of the enhanced ultrasonography using an ultrasound contrast medium, and relates to an ultrasonic diagnostic apparatus and an image processing apparatus capable of presenting, as diagnostic information, a microcirculation at the level of capillaries and the microstructure of a vascular flow relatively fast compared with capillaries.
2. Description of the Related Art
An ultrasonic diagnosis is convenient in that beat pulsations of the heart or motions of a fetus can be obtained as a real-time display through a manipulation as simple as placing an ultrasonic probe to the body surface, and a screening can be performed repetitively due to its high safety; moreover, owing to its small system size in comparison with other diagnosis apparatus for X-ray imaging, CT imaging, MRI, etc., the ultrasonic diagnosis apparatus can be moved to a bedside, so that a screening can be readily performed at the bedside. Although the ultrasonic diagnostic apparatus varies with kinds of functions furnished therewith, the one small enough for an individual to carry around with one hand has been developed, and different from X-ray imaging or the like, an ultrasonic diagnosis has no exposure risk. The foregoing advantages enable the use of an ultrasonic diagnosis in the obstetrics department, home medical care, etc.
An intravenous ultrasound contrast medium has been commercialized, and the enhanced ultrasonography is thus being performed in recent years. This method aims to evaluate the dynamic state of a blood flow when examining, for example, the heart or liver by enhancing blood flow signals with the aid of an ultrasound contrast medium injected intravenously. In most of the contrast media, micro bubbles function as the reflection source. Bubbles, being a delicate material by nature, rupture upon ultrasonic irradiation, even at the ordinary diagnostic level, due to the mechanical function, which results in a decrease in signal strength from the scan surface. In order to observe the circulation dynamically in real time, it is therefore necessary to reduce the scan-caused disruption of bubbles relatively, for example, by producing an image through an ultrasonic transmission at a low acoustic pressure. Imaging through such a low acoustic pressure ultrasonic transmission reduces a signal-to-noise ratio (S/N ratio) as well, and various signal processing methods have been proposed to compensate for such a reduction.
Also, by exploiting the characteristic that the contrast medium bubbles rupture as described above, a method as follows has been proposed. That is, the method includes (A) observing the dynamic state of bubbles filling the scan plane under low acoustic pressure irradiation; (B) destroying bubbles within the plane (to be more exact, within the volume being irradiated) by switching the irradiation acoustic pressure to a high acoustic pressure, and (C) observing a way in which bubbles flow into the plane again. This method is disclosed, for example, in JP-A-11-155858, and is generally referred to as the replenishment method.
Incidentally, a representative diagnostic image extracted by the enhanced ultrasonography is roughly divided to two types. One is a diagnostic image of a relatively fast, thick blood vessel, and the other is a diagnostic image for a tiny blood flow at the level of capillaries (in the case of the liver, a blood flow giving rise to perfusion in the sinusoidal space). Problems with these images are that it is difficult to extract micro-vascular branches on the former vascular image, whereas in the latter, although signals from micro vessels are detected, the vascular branches are not extracted due to the limit of spatial resolution and merely an increase in luminance can be confirmed as a domain. In short, both the images fail to extract micro-vascular branches at the intermediate level. Blood flow information at this level, however, indicates the degree of progress of the shunt of vessels, regenerative nodules, etc., and is therefore said to be information of great importance for a differential diagnosis of a diffuse liver disease or a liver cancer.